Display device and display panel device

ABSTRACT

A functional sheet is brought into intimate contact with a front surface of a plasma display panel, and the functional sheet has a structure in which heat diffusion is superior to heat insulation between the plasma display panel and outside air. In addition, a display device includes a controller for controlling a drive voltage pulse train so that power consumption in a unit area in a light emission region within the screen is limited under a set value when one image is displayed.

This is a Continuation of application Ser. No. 11/056,356, filed Feb.14, 2005, now allowed, and claims the benefit of Japanese Application2004-043360, filed Feb. 19, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device having a display paneldevice including a plasma display panel and a functional sheet that isbrought into intimate contact with a front surface of the plasma displaypanel.

2. Description of the Prior Art

Technology development of a plasma display panel (PDP) that is aself-luminous device is directed to a large screen for providing morepowerful display. One of the important tasks for a large screen isweight reduction of the panel.

In general, a display device including a plasma display panel has arigid filter plate having a base of a tempered glass. This filter plateis arranged in front of the plasma display panel with air gap. Thefilter plate has various functions of adjusting a display coloroptically, preventing reflection of external light, shieldingelectromagnetic waves, and shielding near infrared rays concerningdisplaying operation, and it also has a function of protecting theplasma display panel mechanically.

Arranging the filter plate in front of the plasma display panel iseffective for reducing temperature rise in a filter layer due to heatgenerated in the plasma display panel. Air between the plasma displaypanel and the filter plate works as a thermal insulator. However, thereis a drawback that the heat generated in the plasma display panel may beshut inside an enclosure of the display device, which causes temperaturerise of the plasma display panel.

When the plasma display panel becomes high temperature, misdischarge mayoccur easily. In order to prevent misdischarge, it is necessary to setan upper limit of supplied power to a lower value or to provide athermal countermeasure such as a high power cooling fan. As quantity ofgenerated heat increases along with the screen size becoming larger, anappropriate thermal countermeasure has to be conducted.

In addition, as the filter plate has a large weight, it is not desiredfor a large screen of the plasma display panel. In order to reduce aweight of the display device, another structure is suitable in which athin filter having a base of a resin film is glued directly on the frontface of the plasma display panel instead of attaching the filter plate.Japanese unexamined patent publication 2001-343898 discloses a frontfilter that includes a transparent conductive film for a measure againstEMI and a anti-reflection film that is glued on the front side of thefront filter.

When a filter is glued on the front surface of the plasma display panel,the filter is apt to be deteriorated due to heat generated by the plasmadisplay panel. As the front surface of the plasma display panel iscovered with the filter, heat radiation of the plasma display panel maybe impaired. In the conventional structure, however, there was nocountermeasure against heat for both the filter and the plasma displaypanel in a display device in which the filter and the plasma displaypanel are brought into intimate contact with each other.

SUMMARY OF THE INVENTION

An object of the present invention is to reduce a weight of the devicewhile suppressing excessive temperature rise of the filter and theplasma display panel so that a long life of the filter and stability ofa display can be realized. Another object is to prevent overheating ofan outer surface of the device so that safety about outer surfacetemperature during operation can be ensured.

According to an aspect of the present invention, a display deviceincludes a plasma display panel having a screen including a plurality ofcells, a functional sheet having at least an optical filter function tolight emitted from the screen and being brought into intimate contactwith a front surface of the plasma display panel, and a drive voltageoutput circuit for giving the screen a drive voltage pulse train thatgenerates display discharge plural times corresponding to a gradation.The functional sheet has a structure in which heat diffusion is superiorto heat insulation between the plasma display panel and outside air. Inaddition, the display device includes a controller for controlling thedrive voltage pulse train so that power consumption in a unit area in alight emission region within the screen is limited under a set valuewhen one image is displayed.

The inventor found that selecting a thickness of the functional sheet isimportant for determining thermal properties. FIG. 1 shows an overallrelationship between a thickness and a temperature. In the structure inwhich the functional sheet is brought into intimate contact with a frontsurface of a plasma display panel (hereinafter referred to as a surfaceof the panel), if a condition is used for driving in which a temperatureon the surface of the panel rises up to approximately 70° C. in theconventional structure in which a filter plate is arranged in front ofthe plasma display panel via a space, a temperature on a surface of thesheet is substantially kept at an ambient temperature thanks to thermalinsulation property of the sheet if the functional sheet has asufficient thickness that is more than or equal to 20 mm, for example.The surface of the sheet means a front surface of the functional sheet,namely a surface that is the farthest from a rear face that contacts theplasma display panel. If a thickness of the functional sheet is changedwithout changing a heating condition, a temperature on the surface ofthe panel becomes low as the thickness decreases because thermalinsulation is weaken while thermal diffusing property is enhanced.However, a temperature on the surface of the sheet becomes higher thanthe ambient temperature. Here, the inventor noted that when a thicknessof the functional sheet becomes smaller than a predetermined value, atemperature on the surface of the sheet starts to drop. Thispredetermined value is specifically a value of approximately 2-10 mm,which is determined in accordance with a material of the sheet. From theabove consideration, it is preferable to select a thickness of thefunctional sheet to a value less than or equal to 2 mm that can obtainthermal diffusion effect for dropping a temperature on the surface ofthe sheet in order to prevent excessive heat both on the plasma displaypanel and on the functional sheet. In order that the sheet has an impactabsorbing function that is important for protecting a panel under such alimitation of the thickness, it is preferable to use a soft resin layerthat can perform the function by a single layer.

In addition, the inventor found a temperature on the surface of thepanel and a temperature on the surface of the sheet are determinedsubstantially not by total power supplied to the plasma display panelbut by power consumption per a unit area in a light emission regionwithin the screen (this is called a local power density). Namely, whenthe local power density is controlled to an appropriate upper limitvalue, excessive heat can be prevented both on the plasma display paneland on the functional sheet.

The upper limit value of the local power density is selected to satisfythe following conditions. (1) The temperature on the surface of thesheet must be a temperature that does not give a thermal shock to ahuman body for safety when being touched. More specifically, it must notexceed 70° C. (2) In addition, it is desirable that the temperature onthe surface of the panel does not exceed 80° C. for preventingmisdischarge. (3) The above conditions (1) and (2) must be satisfiedeven if the ambient temperature is an upper limit of the permissibleoperating temperature range (40° C., for example).

Adding to the above-mentioned selection of a thickness of the functionalsheet and limitation of the local power density, a layer structure ofthe functional sheet is devised so that a long life of the opticalfunction can be achieved. A layer containing a material that is apt tooccur thermal deterioration (for example, a coloring matter forselecting a wavelength) is arranged as close as possible to the frontsurface so that it is away from the plasma display panel that is a heatsource. Furthermore, the layer that is arranged on the rear side of thelayer that is apt to occur thermal deterioration is made thick, so thattemperature rise of the material that is apt to occur thermaldeterioration can be reduced.

According to the present invention, a weight of the device is reducedwhile excessive temperature rise of the filter and the plasma displaypanel can be suppressed so that a long life of the filter and stabilityof a display can be realized.

In addition, overheating of an outer surface of the device is preventedso that safety about outer surface temperature during operation can beensured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general relationship between a thickness of a sheet and asurface temperature.

FIG. 2 shows an appearance of a display device according to the presentinvention.

FIG. 3 shows a structure of a display panel device.

FIG. 4 shows a first example of a structure of the display device.

FIG. 5 shows a structure of a principal portion of the display device.

FIG. 6 shows a general outline of fixing of a front sheet.

FIG. 7 shows a layer structure of the front sheet.

FIG. 8 shows a conductor pattern of an electromagnetic wave shieldinglayer schematically.

FIG. 9 shows a mesh pitch of the electromagnetic wave shielding layer.

FIG. 10 shows another example of the mesh pitch.

FIG. 11 shows a method for manufacturing a front portion of the frontsheet.

FIG. 12 shows a method for manufacturing a display panel device.

FIG. 13 shows a circuit structure of the display device.

FIG. 14 shows a concept of a frame division.

FIG. 15 shows a general outline of drive voltage waveforms.

FIGS. 16(A) and 16(B) show a general outline of an automatic powercontrol.

FIGS. 17(A) and 17(B) show drive voltage pulse trains.

FIG. 18 shows a second example of a structure of the display device.

FIG. 19 shows a general outline of a plane shape of the display paneldevice.

FIG. 20 shows a layer structure of the front sheet according to a secondexample.

FIG. 21 shows a third example of a structure of the display device.

FIG. 22 shows a fourth example of a structure of the display device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be explained more in detail withreference to embodiments and drawings.

A plasma display panel that is useful as a color display device is apreferable object to which the present invention is applied.Hereinafter, an embodiment will be described in which an AC type plasmadisplay panel is used as a display panel, which has cells of athree-electrode surface discharge structure.

EXAMPLE 1

FIG. 2 shows an appearance of a display device according to the presentinvention. A display device 100 is a flat type display having a 32-inchdiagonal screen 50. A dimension of the screen 50 is 0.72 meters in thehorizontal direction and 0.40 meters in the vertical direction. A facingcover 101 that defines a plane size of the display device 100 has anopening that is larger than the screen 50, so that a front face of adisplay panel device 1 is exposed in part.

FIG. 3 shows a structure of the display panel device. The display paneldevice 1 includes a plasma display panel 2 that is a device thatconstitutes a screen and a front sheet 3 that is glued directly on thefront face of the plasma display panel 2 to be a display face. The frontsheet 3 is a sheet-like structure corresponding to a functional sheetaccording to the present invention. The plasma display panel 2 is aself-luminous type device that emits light by gas discharge, whichincludes a front face plate 10 and a rear face plate 20. Each of thefront face plate 10 and the rear face plate 20 is a structural elementhaving a base of a glass plate having a thickness of approximately 3 mm.There is no limitation of the structure of the plasma display panel 2when embodying the present invention. Therefore, a description of aninner structure of the plasma display panel 2 is omitted here.

FIG. 4 is a cross-sectional cut along 4-4 line in FIG. 2 and shows thefirst example of a structure of the display device. FIG. 5 is anenlarged view of the portion encircled by the dot-dashed line in FIG. 4and shows a structure of a principal portion of the display device. FIG.6 shows an outline of fixing of the front sheet.

As shown in FIG. 4, the display device 100 includes a display paneldevice 1 arranged in a conductive housing 102 to which the facing cover101 is attached. The display panel device 1 is attached to a chassis 105made of aluminum via a thermal conducting adhesive tape 104, and thechassis 105 is fixed to the conductive housing 102 via spacers 106 and107. A driving circuit 90 is arranged on the rear side of the chassis105. A power source, a video signal processing circuit and an audiocircuit are omitted in FIG. 4.

The front sheet 3 is a flexible layered film including a front portion3A having a thickness of 0.2 mm and having a base of a resin film, and arear portion 3B having a thickness of approximately 1.0 mm made of aresin layer that are put on each other, which will be described later.In particular, the thin front portion 3A that is a functional filmhaving a multilayered structure has a good flexibility. The plane sizeof the front sheet 3, more specifically the plane size of the frontportion 3A is larger than the plane size of the plasma display panel 2,so that the peripheral portion of the front portion 3A is positionedoutside the plasma display panel 2. The plane size of the rear portion3B is smaller than that of the front portion 3A and larger than that ofthe screen.

The conductive housing 102 is a metal plate formed in a boxed shapehaving a rectangular rear face, four side faces and a looped front face.It is also a conductive member surrounding the side faces and the rearface of the plasma display panel 2 apart from them (see FIG. 6). Innerrim of the front face of the conductive housing 102 is placed outsidethe plasma display panel 2 viewed from the front.

In the display device 100, the front sheet 3 extends along the plasmadisplay panel 2 substantially in flat, and only the end portion thereofcontacts the front face of the conductive housing 102. A looped pressuremember 103 is disposed in front of the front sheet 3, which issandwiched between the pressure member 103 and the front face of theconductive housing 102 so that the end portion of the front sheet 3 isfixed to the conductive housing 102. Actually, however, the end portionof the front portion 3A of the front sheet 3 is fixed to the conductivehousing 102 as shown in FIG. 5. Here, the front portion 3A includes anelectromagnetic wave shielding layer 320 having a function of preventinghalation. The electromagnetic wave shielding layer 320 is a rear sidelayer of the front portion 3A. A plane size of the front portion 3A isthe same as that of the front sheet 3 and is larger than that of therear portion 3B. Therefore, when the front sheet 3 is fixed to theconductive housing 102, the electromagnetic wave shielding layer 320 isconnected to the conductive housing 102 electrically. The connectionposition thereof is apart from the plasma display panel 2.

As shown in FIG. 5 well, the plasma display panel 2 and the conductivehousing 102 are connected to each other via a bridge portion 3Aa of thefront sheet 3. As the front sheet 3 has flexibility, a force that isapplied to the plasma display panel 2 can be relieved by deformation ofthe portion 3Aa when a relative position between the plasma displaypanel 2 and the conductive housing 102 is varied due to an impactpressure or heat. An influence on the connection between the front sheet3 and the conductive housing 102 is also reduced. The deformationincludes bending, contraction, expansion and twist.

As a method of fixing the end portion of the front sheet 3, it ispreferable to use a plastic rivet 150 for mass production and reducing aweight. It is preferable that the front sheet 3, the conductive housing102 and the pressure member 103 are provided with holes 3Ah, 102 h and103 h, respectively in advance, which are adapted to the rivet 150.Punching process can make many holes at the same time. Although aprotrusion corresponding to a thickness of the pressure member 103 maybe generated at the end portion of the front sheet 3, increase of athickness of the display device 100 due to the protrusion is onlyapproximately 1-2 mm.

FIG. 7 shows a layer structure of the front sheet. The front sheet 3 isa layered film having a thickness of approximately 1.2 mm including anoptical film layer 310 having a thickness of 0.1 mm, an electromagneticwave shielding layer 320 having a thickness of 0.1 mm, an impactabsorbing layer 351 having a thickness of 1.0 mm, and an adhesive layer352 having a thickness of a few microns in this order from the frontface side. The optical film layer 310 and the electromagnetic waveshielding layer 320 constitute the front portion 3A, and the plane sizesof them are the same. A visible light transmittance of the entire frontsheet 3 is approximately 40% after spectral luminous efficiencycorrection. The impact absorbing layer 351 and the adhesive layer 352constitute the rear portion 3B. A weight of the front sheet 3 isapproximately 500 grams, so the front sheet 3 is much lighter than theconventional filter plate (approximately 2.5 kilograms) for 32-inchscreen.

The optical film layer 310 includes a film 311 made of a PET(polyethylene terephthalate), a anti-reflection film 312 that is coatedon the front side of the film 311, and a coloring layer 313 that isformed on the rear side of the film 311. The anti-reflection film 312prevents reflection of external light. However, the function of theanti-reflection film 312 may be changed from AR (anti reflection) to AG(anti glare). The anti-reflection film 312 includes a hard coat forincreasing scratch resistance of the surface of the sheet up to pencilhardness 4H. The coloring layer 313 adjusts visible light transmittanceof red (R), green (G) and blue (B) for a color display and cuts off nearinfrared rays. The coloring layer 313 contains an infrared absorptioncoloring matter for absorbing light having a wavelength within the rangeof approximately 850-1100 nm, a neon light absorption coloring matterfor absorbing light having a wavelength of approximately 580 nm and acoloring matter for adjusting visible light transmittance in a resin. Anexternal light reflection factor of the optical film layer 310 is 3%after the spectral luminous efficiency correction, and the visible lighttransmittance is 55% after the spectral luminous efficiency correction.In addition, the infrared transmittance is 10% as an average in thewavelength range.

The electromagnetic wave shielding layer 320 includes a film 321 made ofPET and a conductive layer 322 having a thickness of 10 microns that isa copper foil with a mesh portion. The visible light transmittance of anarea of the conductive layer 322 that overlaps the screen is 80%. As thefront surface of the conductive layer 322 is black, the electromagneticwave shielding layer 320 looks substantially coal-black when it isviewed through the optical film layer 310.

The film 311 of the optical film layer 310 and the film 321 of theelectromagnetic wave shielding layer 320 have a function of preventing aglass plate of the plasma display panel 2 from scattering when it isbroken in an abnormal situation. In order to realize this function, itis preferable that a total thickness of the film 311 and the film 321 is50 microns or more. In this example, a total sum of the thickness of thePET is more than or equal to 150 microns.

The impact absorbing layer 351 is made of a soft resin of an acrylicsystem, and a visible light transmittance thereof is 90%. The impactabsorbing layer 351 is formed by applying the resin. When the resin isapplied, it enters spaces of the mesh of the conductive layer 322, sothat the conductive layer 322 becomes flat. Thus, scattering of lightthat may be generated by unevenness of the conductive layer 322 can beprevented.

The impact absorbing layer 351 made of the soft resin contributes tothinning of the front sheet 3. A test was conducted in which the displaypanel device 1 was placed on a horizontal hard floor, and an iron ballhaving a weight of approximately 500 grams was dropped on the center ofthe screen. An impact force just before the plasma display panel 2 wasbroken was approximately 0.73 J. When the plasma display panel 2 withoutthe front sheet 3 was tested under the same condition, the result wasapproximately 0.13 J. When the display panel device in which only theoptical film layer 310 was glued on the plasma display panel 2 wastested under the same condition, the result was approximately 0.15 J.Namely, an improved portion of the shock resistance due to the frontsheet 3 is approximately 0.6 J, and most of the improvement that isapproximately 0.58 J is obtained by the impact absorbing layer 351. Theimpact absorbing layer 351 having a thickness of 1.0 mm is practical.

In addition, the impact absorbing layer 351 prevents plastic deformationof the front portion 3A. If a local pressure is applied by a pen tip orthe like on the above-mentioned PET layer having a function ofpreventing scattering, the portion may be broken and the deformationlooks like a white scar. If the impact absorbing layer 351 is disposedbehind the PET layer, elasticity of the impact absorbing layer 351 mayincrease resistance of the PET layer to breaking. Softness of the impactabsorbing layer 351 may cause a swell of thickness of the entire frontsheet 3. However, as the front portion 3A made of a relatively hardmaterial relieves this swell, the impact absorbing layer 351 hardlyaffects the flatness of the surface of the front sheet 3.

In this example, a rear side surface portion of the resin layer thatconstitutes the impact absorbing layer 351 works as the adhesive layer352. The impact absorbing layer 351 has relatively strong adhesivenessto the electromagnetic wave shielding layer 320 made of PET and copper.On the contrary, the adhesive layer 352 has loose adhesiveness to theglass surface that is the front face of the plasma display panel 2. Theadhesion force thereof is approximately 2N/25 mm. When the front sheet 3is peeled, the front portion 3A is not separated from the rear portion3B so that the front sheet 3 is separated from the plasma display panel2 normally. “Normally” means that an even peeled surface without avisible remaining matter can be obtained.

FIG. 8 shows a conductor pattern of the electromagnetic wave shieldinglayer schematically. The conductive layer 322 of the electromagneticwave shielding layer is an integrated layer of a conductive mesh 322Athat is put on the screen 50 and a looped conductive member 322Bsurrounding the conductive mesh 322A. A plane size of the conductivemesh 322A is larger than that of the screen 50. A width of four sidesconstituting the conductive member 322B is approximately 30 mm. The rearportion 3B of the front sheet is arranged so that the rim thereofoverlaps the looped conductive member 322B along the entire length.Thus, the rim of the rear portion 3B is hidden behind the conductivemember 322B when viewed from the front so that an even appearance is notdeteriorated even if the contour of the rear portion 3B is somethingindefinite in shape. In forming the rear portion 3B, high accuracy isnot required although the peripheral portion of the conductive member322B must be exposed. A variation of approximately 10 mm can bepermitted.

Note that although the conductive mesh 322A is drawn to be coarse inFIG. 8, an actual mesh pitch is substantially the same as the cell pitchof the screen 50, approximately 250 microns, for example. It is possibleto form alignment marks and rivet holes in the conductive member 322Bwithout increasing the number of manufacturing steps of the conductivelayer 322. The alignment marks facilitates the work for gluing the frontsheet 3 on the plasma display panel 2.

FIG. 9 shows a mesh pitch of the electromagnetic wave shielding layer. Alattice of the conductive mesh 322A has a square pattern, and cells ofthe mesh are arranged in the direction that is inclined with respect tothe arrangement direction of the cells 51 in the screen 50. An angle ofthe inclination is 55 degrees in this example. The screen 50 includesmany cells 51 that are arranged in an orthogonal manner. A cell pitch Pvin the vertical direction is approximately 390 microns, while a cellpitch Ph in the horizontal direction is approximately 280 microns. Incontrast, a mesh pitch Pm of the conductive mesh 322A is 250 microns.Here, a length Dm between diagonal lattice points of the mesh isapproximately 350 microns, which is shorter than the cell pitch Pv thatis longer one of cell pitches in the vertical direction and thehorizontal direction of the screen 50. By adjusting this pitch and theangle of inclination of the arrangement direction, the state is obtainedin which all the cells 51 and a part of the mesh are overlapped. Namely,the light shield member is arranged in front of all the cells 51, sothat the effect of preventing halation is obtained over the entirescreen 50 substantially in a uniform manner.

FIG. 10 shows another example of the mesh pitch. In FIG. 10, a lengthDm′ between the lattice points in the diagonal direction of theconductive mesh 322A is the same as the cell pitch Pv in the verticaldirection of the screen 50. In this case, all the cells 51 and a part ofthe mesh are overlapped. In order to make the overlap of the cells andthe mesh more uniform, it is better to make the mesh pitch small.However, considering the strength and the electrical conductivity, it isdesirable that a line width of the mesh is more than or equal to 10microns. It is necessary to note that the visible light transmittancemay be too small if the mesh pitch is decreased under the abovecondition.

FIG. 11 shows a method for manufacturing a front portion of the frontsheet. The front portion is manufactured by a roll-to-roll method thatis used for a multilayered film. A film 310R having a structure in whichan optical film layer continues uniformly and a film 320R having astructure in which many electromagnetic wave shielding layer patternsare connected in a row are manufactured in rolls previously. The film310R and the film 320R are drawn out of the rolls thereof and are put oneach other. Thus, a multilayered film 3AR is obtained and wound in roll,which has a structure in which many front sheets are connected in a row.Here, although the film 320R has a specific pattern including a mesh,precise alignment of patterns between the film 310R and the film 320R isnot necessary because the film 310R is uniform in a plan view. Namely,the structure of the front portion 3A includes only one or no nonuniformlayer, which is a condition of applying the roll-to-roll method. As thewidth W of the film 310R is the same as the width W of the film 320R,alignment in the width direction is substantially neglected when puttingthem on each other in the roll-to-roll method. A little difference ofwidths and a little misalignment in the width direction between thefilms can be permitted.

FIG. 12 shows a method for manufacturing the display panel device. Themultilayered film 3AR is drawn out of the above-mentioned roll on whichthe multilayered film 3AR is wound, and a resin 3B′ to be the rearportion is applied on the multilayered film 3AR. This multilayered film3AR is cut by a cutter 550, and the obtained front sheet 3 is glued on apanel module that is placed on a table 500 after being tested. The panelmodule here means the plasma display panel 2 that is attached to thechassis 105. The plasma display panel 2 of the panel module and thefront sheet 3 are integrated to be the completed display panel device 1.As another manufacturing method, it is possible that the multilayeredfilm 3AR is reversed front side rear after the resin 3B′ is applied onthe same so that it is glued on the panel module, and then it is cut.

As the front portion 3A of the front sheet 3 is formed by cutting themultilayered film 3AR, at least one of the length and the width is thesame completely between the optical film layer 310 and theelectromagnetic wave shielding layer 320 that constitute the frontportion 3A. If cutting of the multilayered film 3AR is performed bypunching, the length as well as the width becomes completely the same.

If a foreign matter is found that entered a space between the frontsheet 3 and the plasma display panel 2 after the display panel device 1is completed, manufacturing yield of the display panel device 1 is stillhigh because the front sheet 3 can be reglued. When the structure of thedisplay panel device 1 is adopted, cost reduction by 20% or more can berealized compared with the case where the conventional filter plate isfixed to the front of the plasma display panel 2.

Concerning the device structure, there is a variation in which theconductive housing 102 is divided into the front portion and the rearportion, and the front portion is fixed to the chassis 105 via aninsulator. In this variation, it is possible to reduce cost of the panelmodule by optimal design of the front sheet 3, the plasma display panel2 and the driving circuit board on the common concept as elements of thepanel module.

Hereinafter, countermeasures against heat in the display device 100 willbe described in detail.

The front sheet 3 diffuses heat well from the plasma display panel 2 toair. It is because that a thermal conductivity of the front sheet 3 hasa value between values of thermal conductivities of the glass thatconstitutes the plasma display panel 2 and air. The thermal conductivityof the front sheet 3 can be regarded as the same as that of a resinmaterial that constitutes main volume of the front sheet 3. Thermalconductivity values of an acrylic system resin of the impact absorbinglayer 251 and PET (polyethylene terephthalate) that is a base materialof a multilayered functional film at room temperature (25° C.=298.73K)are approximately 0.27 W·m⁻¹·K⁻¹ and approximately 0.23 W·m⁻¹·K⁻¹,respectively. In contrast, a thermal conductivity value of the glass isapproximately 1 W·m⁻¹·K⁻¹, while a thermal conductivity value of air isapproximately 0.03 W·m⁻¹·K⁻¹. When the front sheet 3 is disposed betweenthe glass and air, heat diffusing action of the plasma display panel 2is enhanced in general due to reduction of thermal resistances atinterfaces although the number of interfaces is increased.

Here, a coloring matter contained in a coloring layer 313 filter of thefront sheet 3 is deteriorated rapidly if it is heated up to atemperature of 80° C. or more. The impact absorbing layer 351 has to besuperior in thermal resistance to the coloring matter. Morespecifically, it is desirable that a variation in the visible lighttransmittance is 5% or less over the entire range of the visible lightwavelength as a result of a heat resistance test that requires to keep80° C. for 500 hours. In the first example, an acrylic system resin isselected as a material of the impact absorbing layer 351, which has thevariation less than 1% as a result of the above-mentioned heatresistance test. Other transparent resin (PET or a smoothing resin) hasresistance against temperature more than or equal to 90° C. In addition,it is necessary to consider usage at a higher temperature under 40° C.that is an upper limit of a standard environment temperature. Morespecifically, it is desirable that a variation in the visible lighttransmittance is 5% or less over the entire range of the visible lightwavelength as a result of a durability test that requires to keep atemperature of 80° C. and a humidity of 90% for 500 hours. Concerningthe acrylic system resin of the impact absorbing layer 351 in the firstexample, the variation is less than 1% as a result of theabove-mentioned durability test.

This heat radiation action of the front sheet 3 and the followingdriving control are combined so as to prevent excessive temperature riseof the plasma display panel 2 and the front sheet 3.

FIG. 13 shows a circuit structure of the display device. The displaydevice 100 has the plasma display panel 2 and the driving circuit 90. Inthe plasma display panel 2, a display electrode X and a displayelectrode Y that constitute an electrode pair for generating a displaydischarge are arranged in parallel with each other. Furthermore, anaddress electrode A is arranged so as to cross the display electrodes Xand Y. The display electrodes X and Y are row electrodes, while theaddress electrode A is a column electrode. The driving circuit 90includes a controller 71, a data conversion circuit 72, a power sourcecircuit 73, a display ratio detection circuit 720, an X-driver 75, aY-driver 76 and an A-driver 77. The X-driver 75 and the Y-driver 76correspond to a drive voltage output circuit of the present invention.

The driving circuit 90 receives frame data Df together with varioussynchronizing signals from an external device such as a TV tuner or acomputer, and the frame data Df indicate luminance levels of threecolors (R, G and B). The frame data Df is stored in a frame memory ofthe data conversion circuit 72 temporarily. The data conversion circuit72 converts the frame data Df into sub frame data Dsf for a gradationdisplay, which are sent to the A-driver 77. The sub frame data Dsf is aset of display data in which one bit corresponds to one cell. A value ofeach bit indicates whether the cell should emit light or not in thecorresponding sub frame, more specifically whether an address dischargeis necessary or not. The A-driver 77 applies an address pulse to anaddress electrode A that is connected to cells in which the addressdischarge should be generated in accordance with the sub frame data Dsf.Application of a pulse to an electrode means to bias the electrode to apredetermined potential temporarily. The controller 71 controls thepulse application and the transmission of the sub frame data Dsf. Thepower source circuit 73 supplies electric power necessary for drivingthe plasma display panel 2 to each of the drivers.

The display ratio detection circuit 720 of the controller 71 counts bitsthat indicate cells to be energized in the sub frame data Dsf so as todetect a “display ratio” for each sub frame. The display ratio is aratio of the number m of cells to be energized to the total number M ofcells in a sub frame that is a binary image (for example, a lightingratio is m/M×100 in a percentage). The controller 71 increases ordecreases the number of application times of sustain pulses for adisplay discharge in accordance with the display ratio detected by thedisplay ratio detection circuit 720, namely it changes a frequency of adrive voltage pulse train. On this occasion, a relationship between thedisplay ratio and the waveform that is stored in a built-in memory 710is referred to.

A general driving sequence of the plasma display panel 2 in the displaydevice 100 is as follows. In a display using the plasma display panel 2,color is reproduced by binary lighting control. Therefore, as shown inFIG. 14, each of sequential frames F_(k−2), F_(k−1), F_(k) and F_(k+1)(hereinafter the suffixes indicating input orders are omitted) thatconstitute an input image is divided into a predetermined number N ofsub frames SF₁, SF₂, SF₃, SF₄, . . . SF_(N-1) and SF_(N) (hereinafterthe suffixes indicating display orders are omitted). Namely, each frameF is replaced with a set of N sub frames SF. Luminance weights W₁, W₂,W₃, W₄, . . . W_(N-1) and W_(N) are assigned to these sub frames SF inthis order. Each of the weights W₁, W₂, W₃, W₄, . . . W_(N-1) and W_(N)defines the number of display discharge times of each sub frame SF.Although a sub frame arrangement is in the order of weight in FIG. 14,other orders can be adopted. In accordance with this frame structure, aframe period Tf that is a frame transmission period is divided into Nsub frame periods Tsf, and one sub frame period Tsf is assigned to eachsub frame SF. In addition, the sub frame period Tsf is divided into areset period TR for initializing wall charge, an address period TA foraddressing and a sustaining period TS for sustaining. The reset periodTR and the address period TA have constant lengths regardless of theweight, while the sustaining period TS has a length that is longer asthe weight is larger. Therefore, the sub frame period Tsf also has alength that is longer as the weight of the corresponding sub frame SF islarger. Among N sub frames SF, the order of the reset period TR, theaddress period TA and the sustaining period TS is the same. For each subframe, initialization, addressing and sustaining of wall charge areperformed.

FIG. 15 shows a general outline of drive voltage waveforms. In FIG. 15,suffixes (1,n) of reference numerals of the display electrodes Yindicate arrangement orders of the corresponding rows, respectively. Theillustrated waveforms are merely one example, an amplitude, a polarityand a timing thereof can be modified variously. A pulse base potentialis not limited to the ground potential but can be an offset potentialsuch as −Vs/2.

During the reset period TR of each sub frame, a ramp waveform pulsehaving a negative polarity and a positive polarity is applied insequence to all the display electrodes X, while a ramp waveform pulsehaving a positive polarity and a negative polarity is applied insequence to all the display electrodes Y for example, so that anincreasing voltage is applied between the display electrodes of allcells. An amplitude of the ramp waveform pulse increases gradually at arate that is small enough for generating a micro discharge. A compositevoltage that is a sum of amplitude values of pulses applied to thedisplay electrodes X and Y is applied to the cell. The micro dischargethat is generated when the increasing voltage is applied a first timecauses an appropriate wall voltage having the same polarity generated inall the cells regardless of a lighted or a non-lighted state in theprevious sub frame. The micro discharge that is generated when theincreasing voltage is applied a second time adjusts the wall voltage toa value corresponding to a difference between the discharge startvoltage and the amplitude of the applied voltage.

During the address period TA, wall charge that is necessary forsustaining is formed only in cells to be energized. All the displayelectrodes X and all the display electrodes Y are biased to apredetermined potential, and a scan pulse Py is applied to one displayelectrode Y corresponding to a selected row every row selection period(every scan period for one row). At the same time as this row selection,an address pulse Pa is applied to only address electrodes Acorresponding to selected cells that should generate an addressdischarge. Namely, in accordance with the sub frame data Dsf of theselected row, a potential of the address electrode A is controlled in abinary manner. In the selected cell, a discharge is generated betweenthe display electrode Y and the address electrode A, which causes asurface discharge between display electrodes. These discharges in seriescorrespond to the address discharge.

Then, during the sustaining period TS, a sustain pulse Ps having anamplitude Vs of approximately 150-180 volts and a rectangular waveformis applied alternately to the display electrode Y and the displayelectrode X. Thus, the drive voltage pulse train having alternatingpolarities is applied between the display electrode X and the displayelectrode Y. When the sustain pulse Ps is applied, a display dischargeof a surface discharge form is generated in a cell where a predeterminedwall charge remains. The number of application times of the sustainpulse Ps corresponds to the weight of the sub frame as described above.

In the above-mentioned driving sequence, the present invention isstrongly pertinent to the sustaining process during the sustainingperiod TS. Most of heat that heats the plasma display panel 2 isgenerated by the display discharge. The heat generated by the displaydischarge is much larger than heat generated by the electrodes due topower loss.

FIGS. 16(A) and 16(B) show a general outline of an automatic powercontrol. An automatic power control (APC) is a technique of realizing adisplay that is as bright as possible and is seen easily while savingpower by utilizing a phenomenon that even if light emission quantity ineach cell is small in a bright display as a whole screen, it is notconspicuous. The automatic power control enables controlling so thatpower consumption does not exceed a permissible limit. If the displayratio is smaller than a constant value (14% for example), the automaticpower control is not performed substantially, and the number of sustainpulses in a display of one frame is set to a maximum number of sustainpulses that can be applied during a time period defined by the frameperiod. Namely, a sustain frequency is a maximum value f_(max). In thiscase, a drive voltage pulse train TPs_(max) as shown in FIG. 17(A) isapplied to the display electrode X and the display electrode Y. Powerconsumption in one cell becomes at the maximum. If the display ratio issmaller than the above-mentioned constant value, power consumption islarger as the display ratio is larger. If the display ratio is theabove-mentioned constant value, the power consumption is an upper limitvalue P_(max) in a permissible range. When the display ratio exceeds theconstant value, the automatic power control function of the controller71 is activated, and the sustain frequency decreases along increase ofthe display ratio. In this case, a drive voltage pulse train TPs asshown in FIG. 17(B) having a frequency lower than the drive voltagepulse train TPs_(max) is applied to the display electrode X and thedisplay electrode Y.

The plasma display panel 2 has the 32-inch diagonal screen 50 asdescribed above, and the area of the screen 50 is 0.288 square meters(0.72 meters×0.40 meters). For this plasma display panel 2, theautomatic power control limits the power consumption up to 150 W. Here,a local temperature on a surface of the glass (a surface of the panel)of the plasma display panel 2 depends on density of power that issupplied for generating the display discharge in the noted region,namely the above-mentioned “local power density”. The local powerdensity becomes the maximum value at a display ratio when the automaticpower control function starts to work, and a temperature rise on thepanel surface at a light emission area in the screen becomes the mostoutstanding when a frame of such a display ratio is displayed. However,if a light emission pattern consisting of cells that generate a displaydischarge are spread out, temperature is dropped a little due to heatdiffusion in the direction along the plane. In the case of the lightemission pattern of approximately 10 cm square or more, temperature onthe surface of the panel rises at substantially the highest rate.

In this example, the automatic power control function starts to work atthe display ratio of 14%. The display ratio is 100% in the entire whitecolor display in which all the cells emit light. Therefore, an area ofthe light emission region corresponding to the display ratio of 14% isapproximately 0.04032 square meters (=0.288×0.14). Although power to besupplied by the power source circuit 73 is 150 watts, total power thatis supplied to the cells to emit light is approximately 95 watts becausethe driving circuit and the electrodes have their losses. In this case,the local power density is approximately 2356 watts per square meter,and the luminance of display is approximately 350 candelas per squaremeter. If the display of the display ratio 14% is maintained in thisstate, the local temperature on the surface of the panel risesexcessively. Therefore, the controller 71 reduces the local powerdensity down to 1500 watts per square meter that is set as a normalmaximum value during five minutes. More specifically, the sustainfrequency is reduced so that the input power drops from 95 watts toapproximately 60 watts. When the local power density is 1500 watts persquare meter, the luminance of display is approximately 220 candelas persquare meter. If the display of the display ratio 14% is maintained inthis state, rising of the local temperature is saturated inapproximately two hours.

Note that the local power density becomes the normal maximum value alsoin a white color block display having the display ratio of 22% in whichthe automatic power control function works. The final temperature inthis case is similar to the case of the display ratio 14% withdifference within 1° C. because an area of the light emission region issufficiently large.

In the display device 100 of this example in which the above-mentionedcountermeasures against heat were conducted, a temperature on thesurface of the sheet rose to 53° C. as a stationary state under thecondition of the room temperature 25° C. in the white color blockdisplay of the display ratio 14% that is the strictest from a viewpointof a degree of temperature rise. Under the condition of 40° C. that isthe upper limit temperature in the permissible environment of a typicalspecification, the temperature on the surface of the sheet reached 64°C. As the front sheet 3 did not become in an overheated state, anydeterioration of optical characteristics of the filter was not found inthe test even after continuous display during 1000 hours. As a heatradiation property of the front sheet 3 is designed well, a differencebetween the temperature on the surface of the panel and the temperatureon the surface of the sheet was substantially within 10° C. Namely, thehighest temperature on the surface of the panel was 75° C. under thecondition of room temperature at 40° C. This is lower than 80° C., solittle misdischarge was generated.

EXAMPLE 2

FIG. 18 shows a second example of a structure of the display device. Abasic structure of the display device 200 is the same as theabove-mentioned display device 100 except for a screen size. In FIG. 18and the following diagrams, structural elements denoted by the samereference numerals as in FIG. 4 are the same structural elements as ofthe display device 100.

The display device 200 has a 42-inch diagonal screen. A dimension of thescreen is 0.92 meters in the horizontal direction and 0.52 meters in thevertical direction. The display panel device 5 that is a screen moduleof the display device 200 includes a plasma display panel 4 and a frontsheet 6. The plasma display panel 4 includes a front face plate 11 and arear face plate 21, and the front sheet 6 includes a front portion 6Aand a rear portion 6B.

In the display device 200, a plane size of the front portion 6A islarger than the above-mentioned example, four sides of the front portion6A are bent to the rear side substantially in perpendicular manner, andend portions of the front portion 6A are fixed to a conductive housing202. The fixing is performed by sandwiching the front portion 6A betweena side face of the conductive housing 202 and a looped pressure member203. The fixing position thereof is in rear of the front surface of theplasma display panel 4 and is away from the plasma display panel 4. Inthe fixing position, the electromagnetic wave shielding layer of thefront portion 6A and the conductive housing 202 contact each other sothat they are connected in conductive manner.

When the front portion 6A is bent, the fixing position becomes closer tothe plasma display panel 4 than the case where it is not bent so that aplane size of the conductive housing 202 can be reduced. In addition,the fixing position becomes rear more than the case where the frontportion 6A is not bent, so a thickness of the conductive housing 202(size of the side face) can be reduced. Downsizing of the conductivehousing 202 contributes to weight reduction of the display device 200.

Note that if a factory that manufactures the display panel device 5 (adevice manufacturer) and a factory that completes the display device 200by assembling the display panel device 5 in the housing (a setmanufacturer) are separated, it is necessary to prevent the frontportion 6A from being damaged at the peripheral portion duringtransportation of the display panel device 5. For example, when thedisplay panel device 5 is attached to the chassis 205 made of aluminumfor being transported, a package size can be downsized by fixing the endportion of the front portion 6A to the chassis 205 via an insulator.

FIG. 19 shows a general outline of a plane shape of the display paneldevice. The front sheet 6 of the display panel device 5 has notches 61that are formed on four corners of the front portion 6A so as tofacilitate the bending process of the front portion 6A. In addition,plural holes 6Ah are formed along the rim of the front portion 6A, andthe holes 6Ah are used for fixing the front portion 6A.

FIG. 20 shows a layer structure of the front sheet according to a secondexample. A layer structure of the front sheet 6 is the same as theabove-mentioned front sheet 3 except for a thickness of the rearportion. The front sheet 6 is a layered film having a thickness ofapproximately 0.7 mm including an optical film layer 610 having athickness of 0.1 mm, an electromagnetic wave shielding layer 620 havinga thickness of 0.1 mm, an impact absorbing layer 651 having a thicknessof 0.5 mm, and an adhesive layer 652 having a thickness of a few micronsin this order from the front face side. The optical film layer 610 andthe electromagnetic wave shielding layer 620 constitute the frontportion 6A, and plane sizes of these layers are the same. The impactabsorbing layer 651 and the adhesive layer 652 constitute the rearportion 6B. A visible light transmittance of the entire front sheet 6 isapproximately 40% after spectral luminous efficiency correction. Aweight of the front sheet 6 is approximately 500 grams and is muchlighter than the conventional filter plate of the same size(approximately 4.2 kg).

The optical film layer 610 includes a film 611 made of PET, aanti-reflection film 612 that is coated on the front face side of thefilm 611 and a coloring layer 613 that is formed on the rear side of thefilm 611. The anti-reflection film 612 includes a hard coat thatenhances scratch resistance of the surface of the sheet up to pencilhardness 4H. The coloring layer 613 adjusts visible light transmittanceof red (R), green (G) and blue (B) for a color display and cuts off nearinfrared rays. An external light reflection ratio of the optical filmlayer 610 is 3% after the spectral luminous efficiency correction, and avisible light transmittance is 55% after the spectral luminousefficiency correction. In addition, a transmittance of infrared rays is10% as an average within the wavelength range.

The electromagnetic wave shielding layer 620 includes a film 621 made ofPET and a conductive layer 622 having a thickness of 10 microns that isa copper foil with a mesh portion. The visible light transmittance of anarea of the conductive layer 622 that overlaps the screen is 80%.

The impact absorbing layer 651 is made of an acrylic soft resin, and avisible light transmittance thereof is 90%. The impact absorbing layer651 is formed by applying the resin. When the resin is applied, itenters spaces of the mesh of the conductive layer 622, so that theconductive layer 622 becomes flat. Thus, scattering of light that may begenerated by unevenness of the conductive layer 622 can be prevented.

In order to confirm the function of the impact absorbing layer 651, atest was conducted in which the display panel device 5 was placed on ahorizontal hard floor, and an iron ball having a weight of approximately500 grams was dropped on the center of the screen. An impact force justbefore the plasma display panel 4 was broken was approximately 0.40 J.When the plasma display panel 4 without the front sheet 6 was testedunder the same condition, the result was approximately 0.13 J. When thedisplay panel device in which only the optical film layer 610 was gluedon the plasma display panel 4 was tested under the same condition, theresult was approximately 0.15 J. Namely, an improved portion of theshock resistance due to the front sheet 6 is approximately 0.26 J, andmost of the improvement that is approximately 0.24 J is obtained by theimpact absorbing layer 651. The impact absorbing layer 651 having athickness of 0.5 mm is practical.

EXAMPLE 3

FIG. 21 shows a third example of a structure of the display device. Astructure of a display device 300 is substantially the same as theabove-mentioned display device 200.

The display device 300 is characterized in that the inner rim of thefront face of the facing cover 301 is close to a screen area, and soundabsorbing members 351 and 352 are arranged between the facing cover 301and the front sheet 6. The sound absorbing members 351 and 352 are gluedon the facing cover 301 in advance, and the display panel device 5 iscovered with the facing cover 301 so that the sound absorbing members351 and 352 are pressed onto the front sheet 6. As the sound absorbingmembers 351 and 352 are flexible sponge, no excessive force is appliedto the plasma display panel 4. As audible sound noises due to vibrationof the plasma display panel 4 (called an abnormal sound) increases at aperipheral portion of the plasma display panel 4, the noises can bereduced substantially by arranging the sound absorbing members 351 and352. Although the abnormal sound can be shielded by the filter plate inthe conventional structure in which the filter plate is arranged infront of the plasma display panel, the sound can be reflected by thefilter plate and propagate from the rear side to the front side. On thecontrary, as the abnormal sound is absorbed substantially completely inthe display device 300, a quiet display environment can be obtained.Sounds generated by the plasma display panel 4 propagate along the rearportion 6B that is glued on the plasma display panel 4, so it ispreferable to arrange the sound absorbing members 351 and 352 to overlapthe rear portion 6B.

EXAMPLE 4

FIG. 22 shows a fourth example of a structure of the display device. Astructure of the display device 400 is substantially the same as theabove-mentioned display device 300. The display device 400 ischaracterized in that the conductive housing 402 includes a frame-likestructure 402A that is a front portion thereof and a box-like structure402B that is a rear portion thereof. The structure 402A is fixed to thechassis 105 via insulator spacers 403 and 404, and a rim portion of thefront sheet 6 is fixed to the structure 402A via the pressure member203. The structure 402B and the facing cover 301 are attached to thepanel module in which the display panel device 5, the chassis 105 andthe structure 402A are integrated. When attaching the structure 402B,connection members 405 and 406 are used for securing conductiveconnection between the structure 402A and the structure 402B.

In the fourth example, cost of the panel module can be reduced byoptimal design of the structural elements of the panel module on thecommon concept. In a manufacturing form that a device manufacturer and aset manufacturer complete the display device 400, it is possible toincorporate the entire or a part of the electric circuit including apower source into the panel module, or it is possible that the setmanufacturer attaches a part or the entire of the electric circuit tothe panel module together with the facing cover 301.

According to the above-mentioned first through fourth examples, halationcan be reduced more than the case where the front sheet 3 or 6 is notglued. More specifically, a white color pattern of an approximately 10cm square was displayed at a luminance of 350 candela per square meter,and a length from the end of the white color pattern to the end of therange in which light emission having a luminance of 1 candela or moreper square meter appears was measured as an indicator of expansion ofthe halation. When the front sheet 3 or 6 was glued, the halation wasreduced by 0.7 times. Note that when the conventional filter plate isdisposed in front of the plasma display panel away from the panel frontface by 1 cm, the halation is increased by 2.5 times compared with thecase where the filter plate is not arranged.

According to the above-mentioned first through fourth examples, in theconductive layer 322 of the electromagnetic wave shielding layer 320,the conductive mesh 322A that passes light and the looped conductivemember 322B surrounding the conductive mesh 322A are formed integrally,so cost of the display panel device 1 or 5 can be reduced compared witha structure in which a conductive tape is attached around the mesh madeof woven conductive fibers.

It is possible to reduce content of the coloring matter for controllingthe transmittance in the optical film layer 310 or 610 by reducedportion of the visible light transmittance thanks to the electromagneticwave shielding layer 320 or 620, so that the optical film layer 310 or610 hardly generates aging deterioration. According to an acceleratedtest in which the front sheet 3 or 6 is kept in a temperature of 80° C.or more, sufficient optical characteristics are obtained after 20000hours or more.

The rear portion 3B or 6B is sufficiently thick compared with the frontportion 3A or 6A, so deviation of temperature inside the front portion3A or 6A can be relieved. Therefore, a degree of thermal deteriorationof the coloring matter contained in the front portion 3A or 6A can beuniformed.

According to the above-mentioned first through fourth examples, the endrim of the front portion 3A or 6A of the front sheet 3 or 6 protrudesfrom the rear portion 3B or 6B by 1 cm or more, and this protrudingportion can be used as a grip portion for peeling. Namely, the frontsheet 3 or 6 can be peeled easily from the plasma display panel 2, andit is not necessary to spend much cost for a machine for the peelingwork.

According to the above-mentioned first through fourth examples, thefront sheet 3 or 6 having a hard front side and a soft rear side isarranged on the display surface to be in intimate contact, it ispossible to realize mechanical protection of the plasma display panel 2or 4, protection of the sheet itself from damage, and improvement ofdisplay quality due to flatness of the front surface.

According to the above-mentioned first through fourth examples, theimpact absorbing layer 351 or 651 that is also the adhesive layer 352 or652 has a sufficient thickness of 0.1 mm or more, so micro foreignparticles that may exist on the surface of the plasma display panel 2 or4 hardly cause lift of the front sheet 3 or 6. In an ordinary cleanbooth (at atmospheric pressure), plasma display panel 2 or 4 is broughtinto intimate contact with the front sheet 3 or 6 in sufficiently stablemanner. When the plasma display panel 2 or 4 was operated at atemperature of 40° C. and under a reduced pressure corresponding to analtitude of 2000 meters that is the strictest environment for usage ofthe display device, no air bubble or peeling was generated at theinterface between the plasma display panel 2 or 4 and the front sheet 3or 6 even at the condition where a temperature on the surface of thefront sheet 3 or 6 became 64° C. and a temperature on the surface of thepanel became 67° C.

According to the above-mentioned first through fourth examples, a lightand safe display device can be realized by gluing the front sheet 3 or 6directly on the plasma display panel 2 or 4. The highest temperature onthe surface of the sheet can be set to a value lower than the lowesttemperature 70° C. that may give a human body a thermal shock when beingtouched. When a flame is applied onto the surface of the sheet, theflame is not expanded because thermal conductivity between the plasmadisplay panel 2 or 4 and the front sheet 3 or 6 is good. The displaydevice 100, 200, 300 or 400 is also superior in flame retardantproperties. More specifically, when a flame of a gas burner was appliedonto the surface of the sheet perpendicularly for 30 seconds, it startedburning without a flame and the burning stopped within the area ofapproximately 20 mm. When a thickness of the rear portion 3B or 6B was1.2 mm, no burning occurred in the same condition.

The above-mentioned embodiment has the following variations.

It is possible to extend the longest time from five minutes toapproximately 30 minutes for permitting that the local power density ofthe power supplied to the plasma display panel 2 or 4 exceeds the normalmaximum value. Even if the longest time is extended, it is rare that atemperature on the surface of the panel exceeds the highest temperaturedefined by the specification. However, in a particular case where thesame pattern is continuously displayed, there is a potential that atemperature on the surface of the panel exceeds the highest temperature.Therefore, it is preferable that the above-mentioned the longest time isset to a short time. Here, a temperature distribution on the screen canbe predicted by calculation in accordance with a heat diffusing speed onthe surface of the panel and the power density distribution on thescreen. It is possible to install a program for such a prediction intothe controller and to control excessive quantity of the power densityabove the normal maximum value and an exceeding time so that a displayis always performed at the maximum brightness within the range where thefront sheet 3 or 6 does not exceed the highest temperature of thespecification.

If a main material that constitutes the front sheet 3 or 6 has a thermalconductivity ratio more than or equal to 0.1 W·m⁻¹·K⁻¹ and less than orequal to 0.4 W·m⁻¹·K⁻¹ at the room temperature, the same effect as theabove-mentioned example using PET and an acrylic system resin can beobtained concerning the temperature control.

As the electromagnetic shielding layer 320 or 620 having translucencyand electric conductivity, a silver multilayered film can beincorporated instead of the mesh. As the silver multilayered film has afunction of cutting off infrared rays, an infrared absorbing coloringmatter is not necessary when forming the optical film layer 310 or 610.Concerning the coloring layer 313 or 613, a multilayered structureincluding a plurality of layers containing different coloring matterscan be adopted instead of a single-layered structure. However, it ispreferable that an arrangement position of the layer containing thecoloring matter be the front portion rather than the rear portion thatis closer to a source of heat so as to reduce an influence of heat.

It is useful to design a red color fluorescent material (for example,(Y, Gd, Eu)PVO4) and a discharge gas (for example, Ne—Xe gas having Xeratio of 5% or more and gas pressure of 500 Torr, and Xe partialpressure of 20 Torr or more) of the plasma display panel 2 appropriatelyso as to reduce quantity of orange color light. If an optical filtercontaining coloring matter of a wavelength range for absorbing orangecolor light selectively becomes unnecessary, heat resistance can beimproved more and cost of the front sheet 3 can be reduced more.

The most rear face of the front sheet 3 or 6 can be formed as anadsorption surface having a self adsorption function. For example, afterforming the impact absorbing layer 351 or 651, a film made of a siliconematerial is formed on the surface of the impact absorbing layer 351 or651. Thus, it is possible to repeat peeling and sticking between thefront sheet 3 or 6 and the plasma display panel 2 many times. This canreduce a loss of the display panel device during manufacturing processand also contribute to maintenance after it is assembled to the displaydevice. It is because that the front sheet can be replaced easily whenit is damaged. It is also possible that only the anti-reflection layer312 or 612 is made as a sheet having the self adsorption function and isglued on the remaining portion of the front sheet 3 or 6. In this case,the anti-reflection layer 312 or 612 may be glued in a step other thanthe step of gluing the remaining portion of the front sheet 3 or 6 onthe plasma display panel 2 or 4, so that a size thereof may be differentfrom a size of the electromagnetic wave shielding layer 320 or 620. Astrength of the adsorption is desirably adjusted so that peeling can bedone only by a force applied in the perpendicular direction, and theadsorption force is desirably 4N/25 mm or less (when peeling speed is 50mm/min).

Instead of a silicone material, an acrylic foam material that is similarto the material of the impact absorbing layer 351 or 651 may be used,and similar effect can be obtained.

Note that a cleaning process such as using water or air injection shouldbe performed prior to gluing the front sheet 3 or 6, if necessary, andsuch cleaning process should also be performed on an adsorption surfacewhen a peeled front sheet is reused.

The adhesive layer 352 or 652 of the rear portion 3B or 6B of the frontsheet 3 or 6 may be a layer of a material different from the impactabsorbing layer 351 or 651. In this case, regardless of a distortion ofthe surface of the plasma display panel 2 or 4, it is desirable to set athickness of the adhesive layer 352 or 652 to a value more than or equalto 100 microns so that adhesiveness to the front sheet 3 or 6 becomesgood.

Although a plasma display panel is exemplified in the above description,the device constituting a screen is not limited to the plasma displaypanel, and the present invention can also be applied to a device inwhich other display panel such as an EL (Electro Luminescence), an FED(Field Emission Display) or a liquid crystal display constitutes ascreen.

The present invention contributes to improvement of display quality andreliability of a display device having a large screen and a lightweight.

While example embodiments of the present invention have been shown anddescribed, it will be understood that the present invention is notlimited thereto, and that various changes and modifications may be madeby those skilled in the art without departing from the scope of theinvention as set forth in the appended claims and their equivalents.

1. A display device, comprising: a plasma display panel having a screenincluding a plurality of cells; a functional sheet having at least anoptical filter function to light emitted from the screen, being broughtinto intimate contact with a front surface of the plasma display paneland having a structure in which heat diffusion is superior to heatinsulation between the plasma display panel and outside air; a drivevoltage output circuit for giving the screen a drive voltage pulse trainthat generates display discharge plural times corresponding to agradation; and a controller for controlling the drive voltage pulsetrain so that power consumption in a unit area in a light emissionregion within the screen is limited under a set value when one image isdisplayed.